Multi band antennas are antennas providing wireless signals in multiple radio frequency bands, i.e. two or more bands. They are commonly used and are well known in wireless communication systems, such as GSM, GPRS, EDGE, UMTS, LTE, and WiMax systems.
Such multi band antennas often comprises antenna arrays which are commonly used for transmitting and/or receiving wireless communication signals, such as Radio Frequency (RF) signals, in wireless communication systems. In this respect, the antenna arrays often comprises a plurality of antenna elements adapted for transmitting and/or receiving in different frequency bands.
FIG. 1 shows an example of a dual band antenna array according to prior art which comprises dual band and single band antenna elements arranged in a row as shown in FIG. 1. The antenna array arrangement in FIG. 1 comprises dual band antenna elements and single band antenna elements alternately arranged in a row in the order: dual band antenna element, single band antenna element, dual band antenna element, single band antenna element, and so on.
The dual band antenna elements are adapted for transmitting and/or receiving in a lower frequency band and in a higher frequency band while the single band antenna elements are adapted for transmitting and/or receiving in the higher frequency band only. The dual band and single band antenna elements are arranged such that the distance between the centres of two adjacent elements transmitting/receiving in the same frequency is often 0.5-1.0 times the wavelength for the centre frequency for the operational frequency band, and typically around 0.8 of that wavelength. That is, the distance between two adjacent single band antenna elements dx is often 0.8 times the wavelength for the centre frequency for the higher frequency band while the distance between two adjacent dual band antenna elements dy is often 0.8 times the wavelength for the centre frequency for the lower frequency band.
Although this type of antenna array arrangement has proved useful in modern wireless communication system, e.g. in base station antennas, they have some drawbacks.
One such drawback is that the prior art arrangements as shown in FIG. 1 require a certain spacing between the elements depending on the operating frequency in use as described above. Therefore, prior art arrangement is suitable for antenna configurations where the centre frequency for the higher frequency band is approximately two times or less than the centre frequency for the lower frequency band. If the centre frequency for the higher frequency band is more than two times the centre frequency for the lower frequency band the distance between the elements for the higher frequency bands becomes to large which may result in undesired grating lobes.
FIG. 2 shows another prior art antenna array arrangement where two different types of antenna elements are arranged in two different arrays. Antenna elements for a lower frequency band (790-960 MHz) are arranged in the left array and antenna elements for a higher frequency band (2.3-2.7 GHz) are arranged in the right array as shown in FIG. 2. Hence, the two arrays together form a dual band antenna array.
The drawback with the configuration in FIG. 2 is that the width of this antenna arrangement is substantial which leads to a bulky large design with considerable weight. Further, such arrangement also suffers from asymmetric horizontal/azimuth radiation as well as directional error.
Hence, there is a need for an improved antenna array arrangement for multi band antennas in the art.